The present invention relates generally to thermal printing apparatus, and more particularly to a system of controlling energization or actuation of a thermal head in a thermal printer for gradational or multi-level density printing of images.
As printing apparatus for printing images (including characters and figures) formed by computer graphics or others is known a thermal transfer type printing apparatus (printer) in which an ink film 1 comprising a polyester-made film 2 having on its surface heat-fusible ink or heat-sublimation ink, together with the recording sheet 4, is interposed between a line thermal head 6 and a platen roller 5 so as to be movable in accordance with rotation of the platen roller 5. The line thermal head comprises a plurality of heating elements (resistors) to generate heat in response to currents successively supplied from an external device whereby ink is transferred from the ink film onto a surface of the recording sheet for printing. The density determining the gradation of an image printed depends on the area of each dot due to the heating element of the line thermal head in the case of using the heat-fusible ink, while depending upon the sublimation amount for each dot in the case of using heat-sublimation ink. Generally, the heat value of the heating element increases as the time that the current passes through the heating element (which will be referred hereinafter to as energization time) becomes longer so that the area of the dot printed with the heat-fusible ink becomes greater and the ink sublimation amount of the dot printed with the heat-sublimation ink becomes larger so as to cause the density of the printed image to become higher.
FIG. 1 is a graphic diagram showing a transfer characteristic of the heat-fusible ink or heat-sublimation ink, where X1 represents a threshold at which point the ink starts fusing or sublimating. As obvious from FIG. 1, the ink transfer characteristic has a S-figure like configuration and thus the printing preferably requires the control to linearize the ink transfer characteristic. For instance, in the interval up to the threshold point X1 (the interval between 0 and X1), each heating element of the line thermal head is pre-heated by means of a batch energization that a constant current is applied to each heating element for a constant time so as to control the rising of the ink density characteristic. Moreover, in the interval from the threshold point X1 to a predetermined point X2 (which allows the printing), the energization time for each heating element is controlled in correspondance with the resistance of each heating element for gradation control in the color printing. However, there is a problem which arises with such a conventional gradation control, in that the heat values of the respective heating elements are different from each other and further the heat-transferring amounts to the recording sheet by the respective heating elements are different from each other. This is due to the fact that the resistances of the respective heating elements are different from each other and the pressing forces of the line thermal head to the ink film and the recording sheet varies at every place. Thus, there is the possibility that in the pre-heating some of the heating elements do not reach the threshold point X1 and some of the other heating elements exceed the threshold point X1, whereby the density difference (density irregularity) can occur in the printed image and further the recording sheet can be stained in the printing. This stain occurs due to the fact that the ink is undesirably transferred or sublimated up to non-printed portions on the recording sheet due to overheating of the heating element in the pre-heating.
Moreover, for improving the quality of the printed image, there is recently a need to more heighten the maximum density of a printed image particularly in multi-color printing. One possible solution is to increase the power to be applied to the thermal head after the concurrent heating or pre-heating. There is a problem, however, in that the applied power is in non-linear relationship to the image-printing density and hence correction is required for providing the linear relationship therebetween whereby limitation is imposed upon increase in the maximum image-printing density under the condition that the number of density levels is kept constant. Another possible solution is to lengthen the time period of the concurrent heating or pre-heating. However, difficulty is encountered to realize a lower density image with a high quality because the basic density inevitably becomes higher. In addition, there is the possibility that the recording sheet is stained when printing.